Conventionally, in a radio communication system combining OFDM and CDMA (hereinafter referred to as “OFDM-CDMA”), the combination of the characteristic of resistance to frequency selective fading that is an advantage of OFDM modulation, and the characteristic of excellent interference resistance through spreading gain that is an advantage of CDMA, results in an ability to implement high-speed, high-quality communications.
OFDM-CDMA methods broadly comprise a time domain spreading method and a frequency domain spreading method. With the time domain spreading method, spread data that have been spread on a chip-by-chip basis by means of a spreading code are arranged in the time direction within the same subcarrier. With the frequency domain spreading method, on the other hand, spread data that have been spread on a chip-by-chip basis are assigned to different subcarriers.
The frequency domain spreading method will now be described. FIG. 1 is a schematic diagram showing the state of digital symbols before OFDM-CDMA processing, and FIG. 2 is a schematic diagram showing the arrangement of chips after OFDM-CDMA processing using frequency domain spreading. With frequency domain spreading, each of N digital symbols constituting a serial data sequence (FIG. 1) is multiplied by, for example, a spreading code with a spreading factor of M, the same value as the number of subcarriers M.
After spreading, the chips, arranged with M chips in parallel, undergo IFFT (inverse fast Fourier transform) processing sequentially, one symbol at a time. As a result, N OFDM symbols for M subcarriers are created. That is to say, with frequency domain spreading, spread chips are arranged on the frequency axis at their respective times (FIG. 2). In other words, spread chips are allocated to different subcarriers.
A sample configuration of a conventional OFDM-CDMA communication apparatus that implements this frequency spreading method is shown in FIG. 3. First, transmitting system 2 of OFDM-CDMA communication apparatus 1 will be described. In the OFDM-CDMA communication apparatus 1, a plurality of transmit signals 1 through k, . . . , (4k+1) through 5k are input to spreaders A1 through A(5k) that perform spreading processing using different spreading codes. The spread signals are added by adders C1 through C5, as a result of which code division multiplexed signals are obtained. In the case shown in FIG. 3, k transmit signals are multicode-multiplexed by each of adders C1 through C5.
The code division multiplexed signals output from adders C1 through C5 undergo parallel/serial conversion by a parallel/serial converter (P/S) 4, and then undergo orthogonal frequency division multiplexing by means of inverse fast Fourier transform processing by an inverse fast Fourier transform circuit (IFFT) 5. By this means, an OFDM-CDMA signal is formed in which spread chips are distributed among a plurality of subcarriers that have a mutually orthogonal relationship, and this OFDM-CDMA signal is transmitted via a radio transmitting section (RF) 10 that performs radio transmission processing such as digital/analog conversion and signal amplification, and an antenna AN.
Next, receiving system 3 of OFDM-CDMA communication apparatus 1 will be described. In OFDM-CDMA communication apparatus 1, an OFDM-CDMA signal transmitted from an OFDM-CDMA communication apparatus with a similar configuration is input to a fast Fourier transform circuit (FFT) 6 via an antenna AN and a radio receiving section (RF) 11 that performs radio reception processing such as analog/digital conversion. FFT 6 executes fast Fourier transform processing on the input signal, and thereby extracts a code division multiplexed signal distributed among a plurality of subcarriers.
A propagation path compensation circuit 7 compensates for phase fluctuations, etc., occurring in the propagation path, based on a known signal such as a propagation path estimation preamble included in the signal. After propagation path compensation, the signal is despread by a despreader 8, and the received signal for that station is extracted from the spread plurality of transmit signals.
FIG. 4 shows the arrangement of OFDM-CDMA signals formed by OFDM-CDMA communication apparatus 1. As can be seen from FIG. 4, radio transmitting apparatus 1 divides 5k transmit signals 1 through 5k into 5 groups, forms code division multiplexed signals on a group-by-group basis, and performs frequency domain spreading of the code division multiplexed signals in subcarriers of different groups.
Specifically, code division multiplexed transmit signals 1 through k are allocated by frequency domain spreading to subcarriers #1 through #m, the same number as the spreading ratio m, code division multiplexed transmit signals k+1 through 2k are allocated by frequency domain spreading to subcarriers #4m+1 through #5m, and so on through to code division multiplexed transmit signals 4k+1 through 5k, which are allocated by frequency domain spreading to subcarriers #m+1 through #2m.
The number of subcarriers need not coincide with the spreading ratio. Here, a case has been shown in which subcarriers are divided into 5 subcarrier groups, and the spreading ratio is made 1/5 the number of subcarriers in order for code division multiplexed signals to be allocated within each subcarrier group. However, the spreading ratio is not limited to this case, and may be set arbitrarily.
In an OFDM-CDMA communication apparatus, it is necessary to increase the degree of signal multiplexing in order to improve spectral efficiency. However, in a multipath environment, for instance, orthogonality between spreading codes is lost and error rate characteristics degrade. This is because multipathing occurs independently in each subcarrier, and therefore inter-chip orthogonality is lost when each spread chip is spread along the frequency axis.
As the degree of signal multiplexing is increased, in particular, interference between spreading codes also increases, resulting in greater degradation of error rate characteristics. Thus, a problem with conventional OFDM-CDMA communication apparatuses is the difficulty of making spectral efficiency compatible with error rate characteristics.